The distribution of return path digital signal levels in cable television (CATV) systems such as a hybrid fiber/coax (HFC) network has typically followed a default standard based on a power-per-Hertz methodology, which simplifies complex issues associated with different services and requirements. The default standard is also easily understood, readily implemented, and effective for most of today's needs. The reason it works is the inherently high signal-to-noise ratio (SNR) of the hybrid fiber coax (HFC) channel obtained with typical approaches for implementing the return path—both analog and digital returns. Even for the worst-case design environment, represented by a completely full return band, the SNR achieved supports the requirements of a typical set of today's services.
More specifically, in this conventional approach the power is allotted to a digital channel is proportional to the bandwidth allotted to that channel. The technique is based on the fact that a fixed amount of available power, driven by the need to operate below the clipping threshold of the return path laser transmitter or analog-to-digital converter (A/D), must be shared by all users. For example, if we assume a 20 dBmV composite drive at the laser input in the node, the power density for a 5-35 MHz return would be:Per-Hz power density=20 dBmV−10 Log(35×106)=−55 dBmV/Hz.
Or, in MHz,Per-Hz power density=20 dBmV−10 Log(35)=5 dBmV/MHz.
Then, a channel using the highest DOCSIS rate of 2560 ksps, which would have a 30 dB bandwidth of about 3.2 MHz (25% excess BW, or alpha=0.25), would be allotted:5 dBmV/Hz+10 Log(3.2) dB-MHz=10 dBmV
This simple calculation is applied to each channel in the reverse path multiplex.
A variable in this setup—aside from the fact that different manufacturers have different input levels to their return laser transmitters—is the amount of headroom that should be preserved below the clipping levels for a fully loaded return band. This is an issue to be determined on both a practical basis (how much do plant characteristics change, how much power should be attributed comfortably for large interferers), as well as a philosophical one. On the philosophical side, decisions must be made regarding the inclusion of forward error correction (FEC) gain in performance margin budgets and with respect to traffic considerations, such as the viewpoint on percent simultaneous usage and guaranteed access.
FIG. 1 shows a power-per-Hz reverse path loading channel line up. The advantages are obvious—the spectrum is not very complex to observe, create, or check for proper alignment. Signal-to-Noise ratio (SNR) is constant across the band, and headroom is inherently available as services are added (i.e. new return channels are activated). The example above can also be used to point out one of the most obvious shortcomings of this approach. The power allotted to the example channel is 10 dBmV. However, being a DOCSIS channel, it could be either a 2560 ksps QPSK channel, or a 2560 ksps 16-QAM channel. These two modulations are about 7 dB different in performance versus SNR for a given bit error rate (BER). Clearly, then, this technique does not optimally align levels. In fact, an even less bandwidth efficient and more robust modulation, such as BPSK or binary FSK, would be granted the same power allotment. These schemes have virtually the same SNR requirement as QPSK, but deliver less throughput for a given bandwidth.
Once again, the reason that the conventional power-per-Hz approach has been effective is because it takes advantage of the fact that the reverse path, as it was designed, provides substantially higher SNR than is required for the types of signals it is currently being asked to transport. It has built-in margin based simply on the quality SNR it can deliver relative to the needs of these basic digital communication signals, and the power-per-Hz takes advantage of this margin to non-optimally set carrier levels that still can maintain adequate performance, in exchange for simplicity of implementation.
However, technology advances and progress in communication systems design will create more opportunities for the cable return path. For example, an increase in traffic and usage of the available spectrum, an increase in spectrum desired to transport, higher levels of modulation sophistication, the desire for higher throughput from the same spectrum, and the desire to achieve throughput closer to theoretical channel capacity, are all practical situations under consideration and in development stages in some cases.
For example, cable modem deployment is in the midst of a major expansion that will more fully utilize the return path. This expansion will employ various standards that have been proposed to allow transparent bi-directional transfer of Internet Protocol (IP) traffic between the cable system Headend and customer locations over the CATV network. Data traffic via DOCSIS compliant modems, as well as DOCSIS-based VoIP are driving the push for reliable, high-performance, bi-directional systems. Streaming applications such as file sharing applications for music and video will drive guaranteed bandwidth needs, as well as alter the time-domain dynamics of return path traffic.
For coax-to-the-home plants, the preferred choice of reverse path signaling is some form of digitally modulated RF carrier, constrained to the bandwidth split between forward and return particular to that system. Because return traffic existed before the DOCSIS specification, and because the return bandwidth can be used for applications that generate other revenue when the DOCSIS demand for channels is satisfied, the spectrum is a composite of different signal types, data rates, and modulation formats, and will likely continue to be.
Higher performance analog lasers, new digital technologies, and increased return bandwidth splits are some example of technological developments aimed towards the same goal of enhancing the return path's capabilities. In addition to new technologies, it is prudent to consider other procedures that can enhance return path performance. With increasingly sophisticated signaling, in some cases augmented by more bandwidth, it is useful to consider a power allocation method that can improve performance. In order to close the gap between the maximum potential capacity of the return path and its actual capacity as currently implemented, a more optimal way to align the varying signal types on that path must be considered.
Accordingly, it would be desirable to provide a more optimal method of allocating power among the channels on the return path of a CATV network, in which the method is independent of deployed infrastructure or plant technology choices.